This application claims the priority of German Application No. 103 15 933.9 filed Apr. 2, 2003, the disclosure of which is expressly incorporated by reference herein.
The present invention relates to a double-apron drafting unit for spinning machines, comprising a delivery roller pair having a delivery nipping line, also comprising an apron pair, arranged upstream thereof and guiding a fiber strand in the area of the delivery nipping line, and forming together with the delivery roller pair a wedge-shaped gap, as well as means for keeping air currents circulating with the delivery roller pair away from the area of the delivery nipping line.
In the case of such double-apron drafting units, in particular when they are applied to so-called air jet spinning, delivery speeds can be reached by which the air currents, caused by the rotating delivery rollers, cannot be ignored. Due to their high rotational speeds, the rotating rollers pull in air which circulates with the rollers. The air currents are cut off in delivery direction of the delivery rollers at the delivery nipping line. Air is transported continuously into the wedge-shaped gap between the apron pair and the delivery nipping line of the two rollers. This air escapes on the one hand out of the wedge-shaped gap in axial direction on both sides, while on the other hand, it flows in the opposite direction to the transport direction of the fiber strand. This results in air vortexes and to disruption in the fiber flow. The air flow causes a certain amount of disruption among the fibers of the fiber strand which exits from the apron pair and travels towards the delivery nipping line. This is demonstrated, for example, by an exponential decrease in the yarn quality values with the increase in the delivery speed.
The as a rule grooved bottom roller of the delivery roller pair exerts a great influence on the negative air currents flowing into the wedge-shaped gap. The undesirable air currents were reduced somewhat, in that the friction coefficient of the bottom roller was increased, not by fluting, but by application of a corresponding surface coating. Independently of the fact that the fluting method has asserted itself in the industry, a surface coating only would not suffice to prevent the pulling in and circulation of air currents at high delivery speeds.
Many efforts have been made to eliminate the disadvantages described above, or least to reduce their effects. In German published patent application 30 39 149 C2 (corresponding U.S. Pat. No. 4,387,487), for example, it is suggested that the exit area of the apron pair, in relation to the delivery nipping line of the delivery roller pair, be placed in a direction diagonal to the traveling path of the fiber strand. The idea was that by means of this misalignment, the fiber path could avoid the above mentioned air currents, thus subsequently being able to reach the delivery nipping line in an area in close proximity to the surface of the nearest roller of the delivery roller pair. These measures were, however, doomed to failure because, of course, the air currents rotating with the delivery roller pair are interrupted at the delivery nipping line and lead in any case to air vortexes.
In German published patent application 39 31 462, another suggestion was put forward in an attempt to lessen the above mentioned disadvantages in that circulating grooves acting as passageways for a throughput of air were applied to the top roller of the delivery roller pair adjacent to the fiber strand. This was to ensure that those fibers, which tended to yield laterally, were drawn together again. This measure could not, however, be put into practice, as a traversing of the fiber strand, required for such an embodiment for protecting the top roller against wear, was not possible.
An embodiment according to European published patent 0 107 828 (corresponding U.S. Pat. No. 4,520,532) attempted to place the apron pair extremely close to—almost in contact with—the delivery roller pair downstream. The apron pair was to function as a screen against the undesirable air currents. It was, however, shown that in practice, a gap measuring less than 0.2 mm was not possible. Boundary layers of the above mentioned air currents have, however, heights which measure less than 0.2 mm, so that the desired aim could only be partly achieved. This measure results, however, in the advantage that the fiber strand is guideless for only a very short way between the apron pair and the delivery nipping line, which produces a noticeable improvement in the yarn quality values.
Furthermore, further suggestions have been made in which the air currents rotating with the delivery roller pair are screened off.
According to the Japanese published patent 62-15649, an attempt is made to encase the delivery roller pair in the area of the wedge-shaped gap between the apron pair and the delivery roller pair. For this purpose thin metal plates are provided, whose contours match those of the delivery rollers and which are deposited in a flexible way in the area of the delivery nipping line on the rollers. These plates have a slight gap, through which the fiber strand is guided. The disadvantage of this embodiment was simply that these screens were located in the wedge-shaped gap area between the apron pair and the delivery nipping line, so that the apron pair must end at a clear distance upstream of the delivery nipping line, which leads to an extended path without guidance for the fibers of the fiber strand.
A similar measure was attempted in U.S. Pat. No. 3,727,391, whereby freely supported “riders” were placed on the rollers in the wedge-shaped gap between the apron pair and the delivery nipping line, which “riders” surrounded, wing-like, the periphery of both rollers of the delivery roller pair over a certain distance. A slit was located in the centre for guiding the fiber strand through. The disadvantage here is also that the apron pair must end at a clear distance upstream of the delivery nipping line.
Finally, in German published patent application 40 03 019, it is known that the rollers of the bottom roller pair are covered over a large extent of their periphery by semispherical covers disposed thereon in a gliding manner. There is also an opening here for guiding the fiber strand through. This measure does indeed destroy the boundary layers of the undesirable air currents, but the above mentioned disadvantage exists here also in that the apron pair must end at a relatively large distance upstream of the delivery roller pair.
It is an object of the present invention in the case of a double-apron drafting unit of the above mentioned type to keep the undesirable air currents away from the delivery nipping line as effectively as possible, without preventing the apron pair being placed as near as possible to the delivery roller pair.
This object has been achieved in accordance with the present invention in that the means for keeping away the air currents circulating with the delivery roller pair are arranged outside of the wedge-shaped gap formed by the delivery nipping line and the apron pair, but in direct proximity to the delivery roller pair and the apron pair.
With the application of the features of the present invention, it is possible to keep the impairing air currents away from the delivery nipping line without hindering the guiding of the fibers between the apron pair and the delivery nipping line. It is ensured, however, in using the above mentioned measures that the means are not too far away from the delivery nipping line, so that obstructed air currents do not have the chance to form again. It has been shown to be practicable when the means for keeping away the circulating air currents are disposed in a peripheral area of approximately 30° to 60° of the rollers of the delivery roller pair, measured respectively from the delivery nipping line.
Using advantageous measures it can now be attempted to provide means for interrupting the boundary layers of the air currents on at least one roller of the delivery roller pair. Such a measure on the bottom roller of the delivery roller pair is, as explained above, particularly important. The boundary layer of the air currents can then be interrupted when the means are placed nearer than 0.2 mm to the respective roller.
The features according to the present invention can be realized in practice in many varying ways.
In one embodiment it is provided that the means are at a preferably adjustable distance in relation to the periphery of the arranged roller. These means could be, for example, a covering comprising a sealing gap which can be reduced down to 0.1 mm if required. The undesirable air currents diminish with increased reduction of the radial sealing gap.
In another embodiment it can be provided that the means come into contact with the periphery of the arranged roller. This naturally gives the most effective results, as the boundary layer of the air currents are most definitely destroyed. A lip touching the arranged roller or a flexibly arranged shoulder can be advantageously involved in this case. Such means could also comprise a slowly rotating roller, which is provided with pockets on its circumference, with which pockets waste fibers could be collected. Because of the slow rotation of the roller, the pockets can be emptied from time to time or be continuously suctioned. The means may alternatively comprise a brush, which is placed at the respective roller. A fine-bristle, contact brush is especially suitable for the top roller of the delivery roller pair.
It has been shown that means having contact with the delivery roller pair can interrupt the boundary layer of the undesirable air currents most effectively, however, this measure has on the other hand the disadvantage in that rotating fibers may also be nipped. For this reason, it is advantageous in a further embodiment of the present invention when the above mentioned means comprise a suction device. Such a suction device, which may be applied on its own for destroying the boundary layers of the circulating air currents, can be applied to all above mentioned variations. A device having contact with the circumferential surface of the delivery roller pair and having a suction device arranged thereto has proven itself to be particularly effective. A suction roller may be provided in this case as a suction device.
These and further objects, features and advantages of the present invention will become more readily apparent from the following detailed description thereof when taken in conjunction with the accompanying drawings.